when my cousin wouldnt let go of a pacifier my aunty told her that fairies come and take it away when they decided that they are big kids and don't need it anymore. She then got her to lay it out on the table for the fairies before she went to bed and then threw it away and put a new barbie with a note from the fairies saying thanks.

You could try a similar thing with your son and the sippy cup. You could also take him shopping and get him to choose his own "big boy" cups.

Use a bottle for pete's sake. I had all three of my children who are now grown on bottles until they were two. Then we boxed them up and left them for the bottle fairy to come and take them to the new babies. The kids really were able to understand and go to a cup then. My kids are all in their middle 20's now. We did that with pacifiers too. One day one of them saw a baby sucking a pacifier that they use to have and they said, "look mama that baby has my pacie'! It was adorable. Sometimes it is more important to get the liquid into the child, don't worry about the instrument.